The present invention relates to a photoelectric conversion device and an image sensor and, more particularly, to a photoelectric conversion device and an image sensor having improved fixed pattern noise characteristics.
Recently, one- or two-dimensional photoelectric conversion devices having a plurality of photodetectors and a source follower, for storing photo-charges generated by the photodetectors in gate of a MOS transistor and outputting voltage signals converted from the photo-charges, integrally formed on a single semiconductor substrate have been developed. There is a photoelectric conversion device which is disclosed in the Japanese Patent Application Laid-Open No. 9-51085, for instance.
As an example of forming photodetectors and a source follower on a single semiconductor substrate, FIG. 4 shows a circuit configuration of a pixel and FIG. 5 shows a cross sectional view taken along a line B-B' in FIG. 4. The source follower is configured with an n-channel MOS (nMOS) transistor.
The gain of the source follower, G.sub.sf, is, EQU G.sub.sf =1/[1+1/(gm.times.r.sub.ds)] (1)
gm: mutual conductance of a driving transistor PA1 r.sub.ds : saturated drain resistance of a load transistor
Therefore, a large gain is obtained when the mutual conductance of a driving transistor is large.
Accordingly, a source follower configured with an nMOS transistor having larger mutual conductance than that of a p-channel MOS (pMOS) transistor is generally used.
In a MOS transistor, when a voltage is applied across a drain and a source while a channel is formed by applying a voltage to a gate, the electric field becomes strong in the vicinity of the drain-side edge of the channel, which sometimes generates new electron-hole pairs due to impact ionization. Most of the carrier generated due to the impact ionization becomes substrate current and absorbed by a reference potential of the semiconductor substrate, however, a part of the carrier recombines. The recombination is accompanied by light mission, and the emitted light further generates new electron-hole pairs in the semiconductor substrate. The carrier generated in this manner becomes a stray carrier which diffuses over the semiconductor substrate. When the stray carrier enters the photodetectors, ghost signals are generated in addition to essential signals generated in proportion to incident light. These ghost signals are a primary factor of fixed pattern noise in a photoelectric conversion device.
The measurement result, by the applicants of the present invention, of substrate current and drain current with respect to gate voltage Vg of nMOS and pMOS transistors is shown in FIG. 6. In FIG. 6, an abscissa shows the absolute value of the gate voltage, and an ordinate shows substrate current and drain current. The substrate current flowing in the nMOS transistor is about 10.sup.4 to 10.sup.5 larger than that in the pMOS transistor, which indicates that more electron-hole pairs are generated due to impact ionization in the nMOS transistor than in the pMOS transistor. Thus, since the fact that more substrate current flows in the nMOS transistor than in the pMOS transistor, stray carrier is more easily generated in a semiconductor substrate of an nMOS transistor than a pMOS transistor.
Further, substrate current in a MOS transistor depends upon the drain-source voltage more than the gate voltage. Experimental results show that the substrate current increases logarithmically with respect to increase in the drain-source voltage. Accordingly, it is determined that generation of stray carrier can be reduced by lowering the drain-source voltage.
Fixed pattern noise, with respect to a signal level, caused by stray carrier entering a plurality of photodetectors is not ignorable as sensitivity of the photodetectors improves.